1. Field
This patent specification relates to a pickup drive unit and information record/playback apparatus with such drive units. The pickup drive unit is in use primarily for driving an optical pickup to either irradiate laser beams on an information recording medium, or detect light beams reflected from the recording medium.
2. Discussion of the Background
A pickup drive unit is known previously as exemplified by FIG. 11. That perspective view of the structure of the pickup drive unit, shows a pickup 61 equipped with several optical elements for irradiating light beams onto an optical disk, a guiding shaft 62 for guiding the pickup 61 during seek movements in the radial direction of an optical disk, a lead screw 63, which is a cylindrical shaft with spiral grooves provided thereon, and a motor 64.
The lead screw 63 rotates by its direct connection to the motor 64, in which the connection is made such that the screw axis of the former coincides with the rotation axis of the latter.
A rack member 65 is fixed to the pickup 61 to suitably transmit the movements of the lead screw 63. The rack member 65 includes a fixing portion 66 with which the rack member 65 is fixed to the pickup 61; a rack portion 67 provided thereon with gear teeth 69 (or gear portion 69) to be engaged with the grooves of lead screw 63; and a connecting portion 68, which is made of an elastic plate deformable in the direction perpendicular to the axis of lead screw 63 for connecting the fixing portion 66 to the rack portion 67.
The fixing portion 66 includes an L-shaped portion with a face of one of the ends thereof attached to the bottom face of the pickup 61, and two narrow elongated portions each extended from the opposing edges of the other end of the L-shaped portion along the direction of the extension of the bottom face. Respective ends of the connecting portion 68 are connected to the ends of the narrow elongated portions in the cantilever fashion, while the other ends of the connecting portion 68 are connected to the rack portion 67.
The face of rack portion 67 opposing the face having the gear portion 69 is situated directly opposing to the face of the fixing portion 66.
In addition, a compressed coiled spring 70 is located between these two faces of rack portion 67 and the fixing portion 66 to thereby be able to press the rack portion 67 against the lead screw 63.
Furthermore, since the gear portion 69 is then engaged with the grooves provided on the lead screw 63, the displacement suitable for the pickup 61 becomes feasible along the axial direction of the lead screw 63.
In the construction noted above, a frictional force which is produced at the contact portion between gear portion 69 and lead screw 63 by the rotation of the latter, results in a bending moment for the plate-shaped connecting portion 68. This gives rise to a force which acts to cause the rack portion 67 to be either brought close to, or removed from, the lead screw 63, depending on the direction of rotation of the lead screw 63.
FIG. 12A is a side view along the direction of the axis of lead screw 63, illustrating the location of the gear portion 69 engaged with the grooves on the lead screw 63. As shown in FIG. 12B, the shape of the rack member 65 is approximately of F-shaped as viewed along the axial direction.
When the lead screw 63 rotates clockwise as viewed along the direction of the screw axis as shown in FIG. 12A, a bending moment M1 is produced in the connecting portion 68 by a frictional force F1. As a result, the rack portion 67 is forced to be removed from the lead screw 63 in the direction illustrated by the arrow A.
In contrast, when the lead screw 63 rotates counterclockwise as viewed along the direction of the screw axis as shown in FIG. 12B, another bending moment M2 is produced in the connecting portion 68 by a frictional force F2. As a result, the rack portion 67 is forced to be pressed to the lead screw 63 in the direction illustrated by the arrow B.
When the lead screw 63 is rotated clockwise at relatively high speed so as to displace the pickup 61 at a high speed, the gear portion 69 tends to ride across the grooves on the lead screw 63 with more ease as the rack portion 67 is forced away from the lead screw 63. To prevent this, the coiled spring 70 has to be compressed harder to increase the force pressing the rack portion 67 against the lead screw 63.
In contrast, when the lead screw 63 rotates counterclockwise at relatively high speed, the rack portion 67 tends to be pressed harder to the lead screw 63.
With the increase in the strength of the force pressing the rack portion 67 against the lead screw 63, the frictional resistance produced at the contact portion between gear portion 69 and lead screw 63 increases.
This results in negative effects such as worsening record/playback signal qualities from the failure of the pickup 61 to be displaced minutely and smoothly. Also, the increased resistance may impede high speed access movements of the pickup 61.
In order to alleviate the above-noted problems, several techniques have been disclosed such as those described in Japanese Laid-Open Patent Applications No. 11-185407 ('407) and 2000-67534 ('534).
The former application '407 describes an improvement wherein the top of the gear portion 69, which engage with the grooves, are each formed in the shape of an arc so that they constitute a part of the circle representing the elastic bending of the arm of the connecting portion 68. As a result, the engagement of the gear portion 69 with the lead screw 63 is always kept constant even under tangential frictional force caused by the rotation of the lead screw 63.
The latter application '534 describes another improvement, wherein the stiffness of the lead screw 63 is increased against the moment in the tangential direction of rotation by supporting the rack member 65 by means of two parallel springs each made of resinous materials that prevent the gear portion 69 from riding across the grooves on the lead screw 63.
However, several problems remain unsolved.
For example, according to application '407, the distance between the gear portion 69 and lead screw 63 is unchanged even after including the bending of the arm portion. As a result, difficulties persist when an error arises in the distance between the pickup 61 and lead screw 63, and/or in the length of the arm portion because the lead screw 63 can no longer press the gear portion 69 with a force of proper magnitude. This gives rise to either looseness or an increase in friction at the contact portion between the gear portion 69 and the lead screw 63.
With respect to the device described in application '534 the resinous springs tend to have increased length and width. This may cause disadvantages and prevent the reduction of the overall size of the pickup unit. This disadvantage is more evident where optical disk units are to be incorporated into note-sized computer devices: It is desirable for the optical disk unit to be smaller in overall size. Preferably its height is reduced and also its width is reduced in the direction of lead screw axis. The optical unit, however, can not be reduced beyond a certain point since the portion of the resinous springs mechanically interferes with the motor for driving the lead screw.